The present invention relates to heat exchangers made of aluminum, for example, for use as condensers, evaporators, radiators or the like.
The term "made of aluminum" as used herein and in the appended claims includes "made of an aluminum alloy."
For example, condensers for use in motor vehicle air conditioners are known which comprise a zigzag flat tube made of aluminum, corrugated fins made of aluminum and arranged between and joined to the adjacent straight tubular portions of the tube, and a joint member made of aluminum and joined to each end of the tube.
The tube used for the condenser is one which is made of JIS (Japanese Industrial Standard) A1100 which is easily extrudable, and the corrugated fins are usually those made of JIS AlN30 which can readily be shaped. The condenser is fabricated, for example, by assembling the tube, corrugated fins and joint members: applying to the assembly a joining slurry consisting essentially of zinc chloride serving as a Lewis acid and a solvent acting thereon as a Lewis base; and thereafter, heating the assembly. The corrugated fins are joined to the tube by a fillet which is substantially composed of zinc produced by heating the joining slurry.
However, with the conventional condenser which is fabricated by joining the components together, the tube has a pitting potential of -0.725 V, and the corrugated fins have a pitting potential of -0.746 V, while the fillet has a pitting potential of -1.05 V. Since the potential difference between the fillet and the corrugated fins is not sufficiently great, the condenser experiences a problem when used for a prolonged period of time in corrosive environments having a high temperature or high humidity or involving salt dam age in spite of being free of service problems under ordinary conditions. As shown in FIG. 3, cracks 5 develop in the vicinity of the interface between the fillet 4 and the tube 1, as well as between the fillet 4 and the corrugated fin 2; thereby permitting separation of the corrugated fin 2 and resulting in a lower heat exchange efficiency. The reason why such cracks develop is presumably for the following reasons, although this has yet to be fully clarified. Since the potential difference between the fillet 4 and the tube 1 or the fin 2 is not very great, little or no current flows therebetween, and the metal on the fillet side dissolves in the form of cations, especially in the vicinity of the interface between the fillet 4 and the tube 1 or the fin 2; thereby causing corrosion to the boundary portions to produce cracks 5 early.